Constructing a dob

What do you have between the triangles and the bar to allow the triangle to wiggle a bit.

A rubber washer.

Connecting the triangles to the link plate:
Bolt, triangle, rubber washer, link plate, rubber washer, wingnut.
These are tightened just a little, but not much. It's the rubber that allows the triangle freedom to tilt.

Connecting the link bar to the frame:
Nylock nut, metal washer, link plate, metal washer, nylock nut, gap, nut, rubber washer, frame, rubber washer, nut
Nylock nuts are used so that they don't loosen easily. The nylock nuts are not tightened against the link plate at all - just a fraction of a mm of a gap away from the plate. This allows the the collimation bolt to be able to turn without trying to turn the link plate with it. All I need to do now is connect all the triangles together so that they stay in the right place relative to one another.

The rubber washers are tightened hard against the frame by the nuts. The rubber grips the nuts onto the frame so that when the collimation bolt is turned, the rubber washers prevent the nuts from turning, just like as if they were welded to the frame. Cheapest weld ever!

Does this make sense to you?

FWIW, I've decided not to bother with springs as I just found it getting too complicated.

It does indeed, I spent a while evening messing about with the few nuts and washers that I have here. on my cell, but am not happy yet. I need a trip to the hardware store to pick up some more supplies.

That rubber washer sounds the business, I had to google "nylock nut" but it all makes sense now.


here is where I am at at the moment. A little bit of fine tuning needed.


I have also cut the focuser board and drilled the struts to mount the spider. Clever me didn't think about the threaded rod running through the struts getting in the way of the spider screws :oops: . I hope the threaded inserts I have ordered fit my tubes, or its back to the drawing board.

Cutting out the base / tailgate tomorrow and attaching the cell. I should have an idea then about how much it all will weigh so I can start planning the rest of the cutting.

Stage 1 almost complete

kydex, threaded inserts, focuser and teflon arrived in post yesterday.
I have all the bits and pieces cut and in roughly in place for the secondary cage . Just needs a bit of sanding and varnishing and have to cut and attach the kydex before final assembly.

Looking very good Trev!
I waiting on the spider to arrive. Can't do much more till then.

After chatting with Phil this evening (thanks Phil), I managed to measure the focal length of the mirror. I used a small torch and a piece of paper. Paper was positioned adjacent to the torch. The purpose of the paper was to capture the reflection of the light from the torch as it got reflected from the mirror. At the centre of curvature, the image would appear sharp on the paper. Inside or outside of the centre of curvature, the image would appear blurry. The distance from the sharp image on the paper to the mirror is twice the focal length. I positioned the mirror standing up against a wall (side of bed actually, with a pillow in front of it in case it decided to fall over!) and aimed the torch at the centre of the mirror. I then moved away from the mirror until such time as the reflection of the light from the mirror reflected onto a piece of paper and gave a sharp image. The result: I got a reading of 3608mm. Therefore, the focal length is approx 1804mm (within about +/- 5mm). This gives a focal ratio of 4.45. Useful info for calculating the remaining dimensions of the scope.

Hey guys, really nice stuff there.

You secondary looks great Trevor. I like your way of connecting the spider. I had no end of problems with the way I was doing it.
The flotation cell looks alot like what I was going to make. Interestingly the Kriege & Berry book recommends nine points for a 12.5" yet Plop suggests that a six-point like yours is better!

Anyway I just got back to being able to go back to my scope last week. Built the Rocker, Ground Board and Bearings on Thursday through Saturday. So much faster to get these bits done now that the truss and secondary are out of the way.
The rocker was going to be quite tall as I originally envisioned being able to use it as a case for the entire scope. I then read about how much more stable a short rocker is and cut them down to size. This meant making larger bearings - I went for 22 inch. Turns out that there would be no way for me to physically be able to carry the rocker and mirror box at the same time anyway - it's much heavier than you'd imagine.

Anyway, after I put it all together I assumed I would be killed by dreaded balance and stiffness problems, but the scope came out perfectly balanced - as long as I don't use my 30mm Moonfish hand grenade EP. Movement on Alt and Az were both quite smooth.

Took it out for first light on Saturday night and couldn't focus on a thing. Only then did I remember cutting the truss tubes with a few inches to spare as recommended in Kriege's book. Chopping off two inches let me focus on a nice view of Cygnus. extra light was leeking in from everywhere as I didn't make any of the baffling yet, so that's the next project - just added the felt on the secondary cage last night. Here's a shot. Will keep ye updated as to how it works out.



Cheers,

Trev